In the case of using a steel material in the fields such as buildings, bridges, ships, offshore structures, construction machinery, tanks, and penstocks, the steel material is made into a desired shape by welding according to the shape of the steel structure. Steel structures are becoming increasingly larger in size in recent years, and the use of stronger and thicker steel materials is growing markedly.
A thick steel plate having a plate thickness of 100 mm or more is typically produced by blooming a large steel ingot produced by ingot casting and then hot rolling the obtained slab. In this ingot casting and blooming process, however, a concentrated segregation area of a hot top portion or a negative segregation area of a steel ingot bottom portion needs to be discarded. This hinders yield improvement, and causes higher manufacturing cost and longer construction time.
On the other hand, in the case of producing a thick steel plate having a plate thickness of 100 mm or more by a process that uses a continuously-cast slab as a raw material, the aforementioned concern does not exist, but the working reduction to the product thickness is low because the thickness of the continuously-cast slab is smaller than the slab produced by ingot casting. Moreover, the general tendency to require stronger and thicker steel materials in recent years has increased the amount of alloying element added to ensure necessary properties. This causes new problems such as center porosity deriving from center segregation and inner quality degradation due to upsizing.
To solve these problems, the following techniques have been proposed to, in a process of producing an ultra-thick steel plate from a continuously-cast slab, compress center porosity to improve the properties of the center segregation area in the steel plate.
For example, Non Patent Literature (NPL) 1 describes the technique of compressing center porosity by increasing the rolling shape ratio during hot rolling of a continuously-cast slab.
Patent Literatures (PTLs) 1 and 2 describe the techniques of compressing center porosity in a continuously-cast slab by, when producing the continuously-cast slab, working the material using rolls or flat dies in a continuous casting machine.
PTL 3 describes the technique of compressing center porosity by performing forging before hot rolling when producing a thick steel plate with a cumulative working reduction of 70% or less from a continuously-cast slab.
PTL 4 describes the technique of not only eliminating center porosity but also reducing the center segregation zone to improve the resistance to temper embrittlement by, when producing an ultra-thick steel plate from a continuously-cast slab through forging and thick plate rolling with a total working reduction of 35% to 67%, holding the center of the plate thickness of the raw material at a temperature of 1200° C. or more for 20 hours or more before forging and setting the working reduction of the forging to 16% or more.
PTL 5 describes the technique of remedying center porosity and center segregation by cross-forging a continuously-cast slab and then hot rolling the slab.
PTL 6 describes the technique relating to the method of producing a thick steel plate having a tensile strength of 588 MPa or more with center porosity being eliminated and the center segregation zone being reduced, by holding a continuously-cast slab at a temperature of 1200° C. or more for 20 hours or more, setting the working reduction of the forging to 17% or more, performing thick plate rolling so that the total working reduction including the forging is in the range of 23% to 50%, and applying quenching twice after the thick plate rolling.
PTL 7 describes the technique relating to the method of producing a thick steel plate excellent in weldability and ductility in the plate thickness direction by reheating a continuously-cast slab having a specific composition to 1100° C. to 1350° C., with a cumulative working reduction of 15% or more and a strain rate of 0.05/s to 3/s at 1000° C. or more.